1. Field of the Invention
This invention relates to satellite communication system antennas, and more particularly, to satellite antennas having plural feed elements for producing multiple beams.
2. Description of the Prior Art
One well known prior art technique for increasing the communications capacity of a satellite communication system involves the utilization of separate information channels which are isolated from one another by diversity of the direction of polarization of the electromagnetic energy which links the satellite with earth ground stations. In satellite communication systems wherein the launched electromagnetic energy is linearly polarized, maximum isolation is achieved between the channels when such linear polarizations are orthogonal to one another. A problem in the prior art systems relates to cross-talk occurring between orthogonally polarized channels especially when the electromagnetic energy is propagated through an atmosphere containing rain droplets or ice crystals.
The prior art has sought to compensate for the cross-coupling effects of differential attenuation and differential phase shift between the respective information channels by incorporating at the receive station complex restoration apparatus. A differential attenuator having a substantially zero net differential phase shift is described in U.S. Pat. No. 3,747,031 which issued to the present inventor on July 17, 1973. Such an attenuator can be used in conjunction with a differential phase shifter which provides a substantially constant differential phase shift between two orthogonally polarized waves in the same frequency band. A differential phase shifter is disclosed in U.S. Pat. No. 3,755,760 which issued on Aug. 28, 1973 to the present inventor. The theory and mathematics which support the use of a differential phase shifter in combination with a differential attenuator is described in an article entitled "Restoring the Orthogonality of Two Polarizations in Radio Communication Systems, I", by T. S. Chu in The Bell System Technical Journal, Vol. 50, No. 9, November 1971, at pages 3063-3069.
It has been determined in the prior art that cross polarization varies with the amount of power radiated in higher order modes. This problem is especially acute in arrangements wherein the antenna feeds operate in more than one common-carrier frequency band. Such plural frequency band operation requires the use of oversized waveguides in the higher frequency bands, thereby rendering the higher frequency bands more susceptible to the excitation of higher order modes. One solution in the prior art to the problem of cross polarization involves the use of a closely spaced wire grid as a quasi-optical polarization diplexer. The wires, which may be copper strips with a thin mylar backing must be oriented in a preferred direction which requires that the wires be perpendicular to the plane of incidence determined by the beam axis and the grid normal. A mathematical analysis and experimental results are disclosed in an article entitled "Quasi-Optical Polarization Diplexing of Microwaves", by T. S. Chu, J. Gans, and W. E. Legg, in The Bell System Technical Journal, Vol. 54, No. 10, December 1975, at pages 1665-1680.
In prior art synchronous satellite communication systems which transmit linear orthogonally polarized signals, each of the linear orthogonally polarized signals is aligned at the antenna to transmit the correspondingly polarized signals with an essentially parallel orientation over the entire field of view of the antenna radiating such signals. With such signal transmission, it is only along one line in the field of view where the electromagnetic energy at the receiving stations is oriented in the local vertical plane or orthogonal thereto. The problem remaining in the prior art is to provide an antenna capable of transmitting linear orthogonally polarized radiated electromagnetic energy which is received across the field of view of the antenna so that the electromagnetic energy arriving at any receiving station within the field of view receives such energy oriented in the local vertical plane or orthogonal thereto. Such orientation has been determined to substantially reduce cross-polarization effects due to the electromagnetic energy passing through precipitation.